In standard methods of obtaining metallic lead from concentrates, the standard procedure has been to treat the lead sulfide concentrates in a blast furnace. However, the pyrometallurgical procedure possesses many disadvantages and drawbacks. Primary among these disadvantages is that the process will result in some major pollution problems such as the generation of sulfur oxide gas along with substantial fuming. The fuming carries with it possible carcinogenic compounds which will contain lead, cadmium, etc. Therefore, it is necessary to provide improved and safer methods for obtaining metals such as lead in metallic or elemental form by methods which will not contribute to pollution of the air or will be safer to operate. The aforementioned lead smelting techniques will consist of roast sintering the lead sulfide concentrate whereby a major portion of the sulfur will be removed followed by melting in a blast furnace to obtain the metallic lead.
In an effort to alleviate the pollution problem, it is necessary to develop new processes for obtaining lead which will be competitive as an alternative to the conventional smelting practices. Prior work in the hydrometallurgical field resulted in developing a non-aqueous processing route whereby lead sulfide concentrates are chlorinated at temperatures above 300.degree. C. to produce lead chloride and volatilized sulfur. However, chlorination at these elevated temperatures will promote formation of volatile chlorides of contaminating elements such as iron, magnesium, aluminum, silicon, and zinc, as well as elemental sulfur which may be present in the lead sulfide concentrate. Other hydrometallurgical processes which have been developed include the use of ferric sulfate as a leach agent. In this method, the lead sulfide is sulfated to form lead sulfate. This step is then followed by carbonation of the lead sulfate to form lead carbonate and thereafter the lead carbonate is subjected to dissolution in hydrofluosilicic acid for electrolysis to metallic lead. Yet another hydrometallurgical method which is developed for the recovery of lead is based on the use of an acidic ferric chloride medium. This method involves a leaching step whereby the lead sulfide is converted to lead chloride and thereafter subjected to steps of solubilizing, crystallization and electrolysis.
The prior art which discloses other processes for converting a lead sulfide to elemental or metallic lead is also exemplified by U.S. Pat. No. 1,491,653. This reference describes the use of chlorides of sulfur and in particular sulfur monochloride to selectively chlorinate lead sulfide in a complex lead-zinc sulfide ore at temperatures ranging from 50.degree. to about 150.degree. C. However, in this method of operation, the solids are reacted in a solution of sulfur monochloride to form a slurry. A disadvantage which is present when utilizing such a system is that certain metal sulfides are solubilized due to the dissolution of sulfur into the slurry, this dissolution being due to the wide range of compositions of sulfur chlorides. Such an action can lead to the dissolution of some metal sulfides thereby rendering the process more complex in nature. In addition to this type of operation, another metal recovery system utilizes a dry chlorination of complex sulfides in a two-stage process. The first stage consists in a countercurrent chlorination of the ore in a tube mill with chlorine gas, the temperatures of this process usually being in a range of from about 100.degree. to about 150.degree. C. to insure chlorination of from about 40% to about 70% of the metals. The important step in this stage is the chlorination of iron which serves as a source of chlorine in the second step. The second step of this two-stage process consists in a chloridizing roast wherein the final chlorination is accomplished to convert all metals present in the ore to chlorides. Much of this reaction is done by the release of chlorine by the oxidation of the initially formed ferric chloride to ferric oxide and chlorine. Following this, the metal chlorides are then leached in water and brines in order to solubilize the metals. However, the chloridizing roast to produce ferric chloride will also produce sulfur and sulfur chlorides as well as oxides, the roast temperatures which are necessary to accomplish this being above 138.degree. C. and probably above 150.degree. C.
One example of the two step process is found in U.S. Pat. No. 4,011,146. In this reference a dry chlorination of a sulfide ore is conducted in which the products which are formed during the aforesaid drying chlorination step are contacted with an inert sweep gas such as nitrogen to convert any sulfur chlorides which are formed to metal chlorides and elemental sulfur. The patent teaches that the lead sulfide present in the ore is converted by an exothermic reaction to lead chloride and, therefore, some cooling must be effected or in the alternative inert materials must be added.
As will hereinafter be set forth in greater detail, it has now been discovered that a lead sulfide source may be subjected to halogenation in the presence of an oxygen-containing gas which is substantially inert to the environment to provide a fluidized bed for the halogenation reaction. By utilizing this fluidized bed, it will be possible to effect a more thorough halogenation of the lead sulfide source with a concomitant increase in the yield of the desired lead.
This invention relates to an improvement in a hydrometallurgical process for the recovery of metallic lead. More specifically, the invention is concerned with an improved process for the halogenation and particularly chlorination of a lead sulfide concentrate wherein the halogenation is effected in a fluidized bed reactor in either a batch or continuous type of operation.
It is therefore an object of this invention to provide an improved process for the halogenation of lead-containing sources.
A further object of this invention is to provide an improvement in the chlorination of a lead sulfide concentrate whereby a more efficient mixing of gas-solids which is a requirement of the reaction is effected.
In one aspect an embodiment of this invention resides in a process for the halogenation of a lead sulfide which comprises halogenating said lead sulfide at an elevated temperature with a halogen gas in a dry atmosphere, the improvement which comprises halogenating said lead sulfide in a fluidized bed with said halogen gas, said fluidized bed being effected by the introduction of a gas consisting essentially of a mixture of said halogen gas and an oxygen-containing gas at a rate of from about 4 centimeters per second to about 12 centimeters per second to said lead sulfide within said fluidized bed, wherein the oxygen in said oxygen-containing gas is substantially inert to said halogenation within said fluidized bed.
A specific embodiment of this invention resides in the process for the halogenation of a lead sulfide in which said lead sulfide is chlorinated at a temperature in the range of from about 90.degree. to about 120.degree. C. with chlorine gas in a dry atmosphere, said halogenation being effected in a fluidized bed operation which is afforded by the introduction of a gas consisting essentially of a mixture of said chlorine gas and air at a rate of from about 4 centimeters per second to about 12 centimeters per second, said chlorine gas being present in said mixture in a ratio of from about 0.01:1 to about 0.2:1 parts by volume of chlorine gas per part of said air.